1. Field of the Invention
This invention relates to wire bonding of semiconductor devices and, more particularly, to a method and apparatus for fine pitch wire bonding using a shaved capillary.
2. Brief Description of the Prior Art
Ball bonding is a widely used technique in semiconductor fabrication to connect the internal semiconductor die to the external leads. In this procedure, a fine gold wire, usually about 25 .mu.m (0.0010 inch) in diameter, is fed down through a ceramic capillary, generally alumina, having an entry aperture at the top and an exit aperture at the opposite end of a bore therein. A ball is formed external to the exit aperture by an electronic flame off (EFO) mechanism that melts a small portion of the wire remaining after the previous bond. Essentially, the ball is formed at the end of the wire by an electric discharge spark. At this time, the capillary is relatively far from the ball (millimeters distant). The wire is restrained from moving by a tensioner until the ball is centered in the chamfer diameter of the capillary and is forced downward by the continued motion of the capillary toward the bond pad on the die. The ball is placed on a bond pad of the semiconductor device being assembled and the capillary end then forces the ball against the pad to provide the bond in conjunction with thermosonic energy.
The above described ball bonding step presents a major obstacle to gold wire ball bonding in integrated circuits with the bond pads closer than approximately 100 .mu.m (0.0039 inch) due to the diameter of the capillary. As the pad-to-pad pitch of semiconductor devices decreases, there is less room for the capillary to make a ball and bond the ball and attached wire to a pad without interfering with the ball and wire on an adjacent pad. Current fine-pitch gold ball bonding uses a fine pitch or "bottlenose" capillary. However, it is not possible to shrink the capillary diameter sufficiently to produce bond pitches below approximately 90 .mu.m (0.0035 inch) without causing stitch bond strength degradation. The reason for this stitch bond strength degradation is that the dimensions of the face of the capillary that forms the stitch bond shrink as the capillary diameter shrinks, thereby reducing the area in which the stitch is formed. Also, great stress is placed on the capillary bond face during the stitch bond. The capillary bond tip is literally forced into the leadframe, leaving an imprint of the capillary tip in the lead finger, thereby presenting the same problem as discussed above with regard to the ball bond. Additionally, capillary cost increases due to reduced manufacturing yields and capillary life is reduced because the capillary with reduced diameter is more fragile than a standard capillary.
A capillary that has had two parallel cuts made to its sides for ball to ball clearance is sometimes used to ball bond very fine pitch semiconductor die. Such a capillary is usually referred to as a "shaved" capillary. Since the parallel cuts remove the face portion of the capillary that makes the stitch bond on either side of the capillary, such a shaved capillary can only be used to bond wires that are more or less directly perpendicular to the die to keep the wire under the remaining face of the capillary for proper stitch bond. This limits the use of a shaved capillary to either all x or all y direction wires with little or no wire angle to the side of the semiconductor die or between a set of wires. The wires for such a capillary have to be substantially parallel as a set which drastically limits the usefulness of such a capillary design. However, the remaining face of such a shaved capillary is much larger than the face of a capillary that can bond in all directions at the same pad-to-pad pitch, and this larger capillary face allows the shaved capillary to make very good stitch bonds, compared to stitch bonds made with a same pad-to-pad pitch, all-direction capillary.
Wedge bonding is another widely used technique in semiconductor fabrication to connect the internal semiconductor die to the external leads. Here, aluminum wire is usually used, but gold wire is an option. Wire sizes are similar to those used in ball bonding. The bonding tool and process is quite different. The wire is fed under a metal bonding tool that makes a "stitch" bond, by pressing a small portion of the wire to the semiconductor's bond pad. The tool is then moved in z and x-y directions to loop the wire to the external lead and make a second stitch bond on the lead. The tool can only pull wire and wires are normally bonded in the y direction of the bond head. Either the part or the bond head has to be rotated to align each bond pad and external lead to the tool path in the y table direction.
The resultant stitch bond is rectangular in shape, with the thin dimension being in the x direction of the resultant bond. Bonds made perpendicular to the direction of the bond pads can make very tight pitch with the external leads and this causes the resultant stitch bond to rotate on its bond pad, increasing the pad to pad pitch required for proper clearance.
It is therefore apparent that an improved technique for making wire bonds to pads of semiconductor devices will be increasingly desirable as the dimensions and spacing of the bond pads shrink.